fishbaits: Not played fourth (or any except first lol).
What makes fourth best one?
It's one of the very few FPS games, and certainly the only Quake game, where I feel like I'm part of an actual conflict. At one point you try to sabotage a certain important Strogg facility, and I'm always surprised to find myself really caught up in the mission because of its importance. Almost hoping each time that somehow we'll succeed against the game's wishes.
It's that kind of atmosphere, together with very solid shooting mechanics and a very well crafted world that, imo, places the game several notches above any other quake game.